1. Field of the Invention
The present invention relates to a radio set for performing radio communications, and more specifically to a frequency conversion system of a radio set using a direct conversion system.
2. Description of the Prior Art
Generally, a radio set for performing radio communications includes a radio unit for converting a frequency, and a base band process unit for modulating or demodulating voice and image data. Conventionally, the method of configuring a radio unit forming a radio set can be a double conversion system for performing a frequency converting process two times, a single conversion system for performing a frequency converting process once, etc.
Assuming that the double conversion system or the single conversion system is operated using a reception system of a radio unit forming a radio set, in the case of the double conversion system, a received high frequency signal is frequency-converted into a received signal on a first intermediate frequency band using a first frequency converter, then the signal is frequency-converted into a received signal on a second intermediate frequency band using a second frequency converter, the resultant signal is orthogonally demodulated by an orthogonal demodulator, and is frequency-converted into a received signal on a base band. Then, the base band process unit demodulates the received signal.
Furthermore, in the single conversion system, a received high frequency signal is frequency-converted into a received signal on an intermediate frequency band by a frequency converter, orthogonally demodulated by an orthogonal demodulator, and frequency-converted into a received signal on a base band. Then, the received signal is demodulated by the base band process unit.
In the radio set in the double or single conversion system, a higher harmonics signal or a distortion signal is generated by nonlinearity of an amplifier, a frequency converter or the like forming a radio set. These higher harmonics signals and distortion signals are interference waves for other radio sets by functioning as unnecessary radiation in a transmission system, and, in a reception system, are interference waves by functioning as interference waves which deteriorate the reception characteristic of the system. Thus, the double conversion system or the single conversion system uses means for removing the interference waves by these higher harmonics signals and distortion signals by using a plurality of band pass filters for passing only necessary signals for transmission.
To realize a smaller and lighter radio set, the double conversion system and the single conversion system require a plurality of band pass filters and so forth as described above, and there is a limit in realizing a radio set smaller and lighter than that having the current configuration. Under the situation, a direct conversion system has attracted attention as a practical system of realizing a smaller and lighter radio set.
In the direct conversion system, a reception system orthogonally demodulates a received signal on a high frequency band, directly performs a frequency-converting process into a received signal on a base band, and then demodulates the signal by a base band process unit. A transmission system orthogonally modulates a transmission signal on a base band, performs a frequency-converting process directly into a high frequency signal, and then transmits it. In the direct conversion system, since no intermediate frequency is used during the radio process performed by a radio unit, a higher harmonics signal or a distortion signal is not generated for a signal on an intermediate frequency band. That is, the unnecessary radiation or interference waves due to the higher harmonics signals or the distortion signals can be reduced as compared with the double conversion system or the single conucersion system. Therefore, it is not necessary to provide a plurality of band pass filters, thereby successfully realizing a smaller and lighter radio set.
Described below will be a radio set in the direct conversion system. First, assuming that the direct conversion system is applied to each of the transmission system and the reception system forming a radio unit of a radio set, when the direct conversion system is applied to a transmission system, the frequency of a local oscillation signal is equal to the frequency of a transmission signal. Therefore causing the problem that the leakage component of the local oscillation signal cannot be removed. On the other hand, when the direct conversion system is applied to a reception system, the frequency of a local oscillation signal is equal to the frequency of a reception signal, thereby generating the leakage component of the local oscillation signal. However, the generated leakage component is equivalent to a direct current voltage component which can be removed at the output terminal of the reception system. Therefore, a radio set in the direct conversion system can be the optimum system when the direct conversion system is applied to a reception system while the single conversion system is applied to a transmission system.
FIG. 7 is an example of a block diagram of a radio set having the above mentioned configuration. As shown in FIG. 7, a direct conversion radio set includes: an antenna 101; a transmission and reception common circuit 102; a high frequency amplifier 103; a band pass filter 104; a variable gain amplifier 105; an orthogonal demodulator 106; a low pass filter 107; a low pass filter 108; abase band process unit 109; a first local oscillator 110; a second local oscillator 111; an isolator 112; a transmission amplifier 113; a band pass filter 114; a high frequency amplifier 115; a band pass filter 116; a frequency converter 117; a band pass filter 118; a variable gain amplifier 120; and an orthogonal modulator 121.
The antenna 101 receives or transmits a signal from or to a base station of radio communications not shown in the attached drawings, and is connected to a transmission and reception input/output terminal of the transmission and reception common circuit 102 for filtering a transmission/reception signal. The reception side output terminal of the transmission and reception common circuit 102 is connected to the input terminal of the high frequency amplifier 103 for amplifying a received signal on a radio band, and the output terminal of the high frequency amplifier 103 is connected to the input terminal of the band pass filter 104 for passing only the reception band of the radio frequency band.
The output terminal of the band pass filter 104 is connected to the input terminal of the variable gain amplifier 105 for controlling a gain depending on the power of a received signal, and the output terminal of the variable gain amplifier 105 is connected to the input terminal of the radio frequency band of the orthogonal demodulator 106 for orthogonally demodulating the received signal of the radio frequency band, and frequency-converting the signal into a received signal of an I component and a Q component of a base band. The input terminal of the local signal of the orthogonal demodulator 106 is connected to the output terminal of the local signal of the first local oscillator 110 for oscillating the first local signal. The output terminals of the I component and the Q component of the orthogonal demodulator 106 are respectively connected to the input terminals of the low pass filter 107 and the low pass filter 108, and the output terminals of the low pass filters 107 and 108 are connected to the input terminals of the reception I and Q components of the base band process unit 109.
The output terminals of a transmission I component and a transmission Q component of the base band process unit 109 are connected to the input terminals of an I component and a Q component of the orthogonal modulator 121 for orthogonally modulating the transmission signal of the base band, and frequency-converting the signal into the transmission signal of the intermediate frequency band. The output terminal of the intermediate frequency band signal of the orthogonal modulator 121 is connected to the input terminal of the variable gain amplifier 120. In addition, the input terminal of the local signal of the orthogonal modulator 121 is connected to the output terminal of the local signal of the second local oscillator 111 for oscillating a second local signal. The output terminal of the variable gain amplifier 120 is connected to the input terminal of the band pass filter 118 for passing only the transmission signal of the intermediate frequency band, and the output terminal of the band pass filter 118 is connected to the input terminal of the intermediate frequency band of the frequency converter 117 for frequency-converting the transmission signal of the intermediate frequency band into the transmission signal of the radio frequency band.
Furthermore, the input terminal of the local signal of the frequency converter 117 is connected to the output terminal of the first local signal output from the first local oscillator 110. The output terminal of the radio frequency band signal of the frequency converter 117 is connected to the input terminal of the band pass filter 116 for removing the higher harmonics and the unnecessary radiation of image components output from the frequency converter 117, and passing only the transmission signal of the radio frequency band. The output terminal of the band pass filter 116 is connected to the input terminal of the high frequency amplifier 115 for amplifying the transmission signal of the radio frequency band.
The output terminal of the high frequency amplifier 115 is connected to the input terminal of the band pass filter 114 for removing the higher harmonics component output from the high frequency amplifier 115, and passing only the transmission signal of the radio frequency band. The output terminal of the band pass filter 114 is connected to the input terminal of the transmission amplifier 113 for amplifying the transmission signal up to the level of the power enough to output the transmission signal from the antenna 101. The output terminal of the transmission amplifier 113 is connected to the input terminal of the isolator 112, and the output terminal of the isolator 112 is connected to the input terminal of the transmission signal of the transmission and reception common circuit 102.
Described below will be the operations of the radio set. First, the operations of the reception system are described. The reception system of the direct conversion radio set receives a signal transmitted from a base station not shown in the attached drawings from the antenna 101. The received signal received by the antenna 101 passes through the transmission and reception common circuit 102, is amplified by the high frequency amplifier 103, passes through the band pass filter 104 and the variable gain amplifier 105, and is input to the orthogonal demodulator 106.
The base band process unit 109 controls the gain for the variable gain amplifier 105 such that the power of the received signal input to the orthogonal demodulator 106 can be constant. The orthogonal demodulator 106 orthogonally demodulates the input received signal by using the first local oscillation signal oscillated by the first local oscillator 110, frequency-converts the received signal of the radio frequency band into the received signal of a base band, and outputs the signal as the received signals of an I component and a Q component. In the process of orthogonally demodulating a signal by the orthogonal demodulator 106, the first local oscillation signal oscillated by the first local oscillator 110 is input to the 90° phase shifter forming part of the orthogonal demodulator 106, thereby generating a first local oscillation signal with the phase shifted by 90°.
Each of the first local oscillation signal and the 90° phase-shifted first local oscillation signal is multiplied by the received signal input to the orthogonal demodulator 106 to generate a received signal of an I component and a received signal of a Q component. The received signals of the I component and the Q component output from the orthogonal demodulator 106 respectively pass the low pass filters 107 and 108, and are input to the base band process unit 109. Then, a base band signal process is performed.
Described below will be the operations of the transmission system. In the base band process unit 109, the transmission signals of the I component and the Q component are generated and input to the orthogonal modulator 121. In the orthogonal modulator 121, the transmission signals of the I component and the Q component input to the orthogonal modulator 121 are orthogonally modulated by using the second local oscillation signal oscillated by the second local oscillator 111, and the transmission signal of the base band is frequency-converted into the transmission signal of the intermediate frequency band, and output as a transmission signal of the intermediate frequency band.
In the process of orthogonally modulating a signal in the orthogonal modulator 121, the second local oscillation signal oscillated by the second local oscillator 111 is input to the 90° phase shifter forming part of the orthogonal modulator 121, and a second local oscillation signal is generated with the phase shifted by 90°. The second local oscillation signal is multiplied by the I component transmission signal input to the orthogonal modulator 121, and the 90° phase-shifted second local oscillation signal is multiplied by the Q component transmission signal input to the orthogonal modulator 121, thereby orthogonally modulating the signal.
The transmission signal of the intermediate frequency band output from the orthogonal modulator 121 passes through the variable gain amplifier 120 and the band pass filter 118, and is input to the frequency converter 117. In the frequency converter 117, the transmission signal of the intermediate frequency band input to the frequency converter 117 is frequency-converted into the transmission signal of the radio frequency band by using the first local oscillation signal oscillated by the first local oscillator 110, and is then output.
The transmission signal of the radio frequency band output from the frequency converter 117 passes through the band pass filter 116, the high frequency amplifier 115, and the band pass filter 114, and is input to the transmission amplifier 113. The transmission signal input to the transmission amplifier 113 is amplified up to the power to be output from the antenna 101, and is then output. The transmission signal output from the transmission amplifier 113 passes through the isolator 112 and the transmission and reception common circuit 102, and is then output from the antenna 101.
With the configuration in which the direct conversion system is applied to the reception system of the radio set while the single conversion system is applied to the transmission system, the frequency used in the radio process will be described below. Assuming that the reception frequency is Frx, the transmission frequency is Ftx, and Frx>Ftx, the intermediate frequency IFtx of the transmission system is expressed as follows.IFtx=Frx−Ftx  (1)Ftx=Frx−IFtx  (2)
When the transmission signal and the transmission signal of the intermediate frequency band leak to the reception system, that is, when Ftx+IFtx leak to the reception system, the following equation exists.Ftx+IFtx=(Frx−IFtx)+IFtx=Frx  (3)
Thus, there arises the problem of an interference wave for a received signal.